Water mass variations in the oceans and estimate of thermal expansion by combining sea level data from satellite altimetry with GRACE ocean data
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GRACE data on the oceans reflects the variations in ocean water masses. When geographically averaged over the ocean domain, the GRACE-derived ocean mass represents the 'water mass' component to sea level variations due to land ice melt and terrestrial water storage. Figure 1 below presents ocean water mass variations derived from GRACE data (in red: GFZ geoids; in black: GRGS geoids) for 2002-2006. They exactly match the total land water storage, observed by GRACE (but with an inverse sign).  |
Direct estimation of thermal expansion - totally independent from ocean temperature measurements - is another application of GRACE. This estimate results from combining GRACE ocean data with Topex/Poseidon sea level measurements. Satellite altimetry measures the sum of the steric and ocean mass contribution to sea level, whereas GRACE only measures the 'water mass' component. The steric component is the difference between these two types of data expressed in terms of mean sea level. Figure 2 below compares estimates of thermal expansion from the combination of GRACE data with Jason-1 data (in red: GFZ geoids; in black: GRGS geoids) and from in situ ocean temperature data (in blue: WOD04 climatology; in green: data from Ishii et al., 2006). These two totally independent estimates are in good agreement. Results are published in Garcia et al. 2007 and Lombard et al. 2007.  |
After removal of the seasonal cycle, the thermal expansion deducted from Jason-1 and GRACE data presents a positive slope since 2003, while in situ hydrographic data indicate a negative slope (cooling of the ocean) (Figure 3). Such a cooling seems a little suspect as coinciding with apparition of temperature data from the ARGO network. According to very recent information, an instrumental problem on certain ARGO buoys could explain a part of the apparent cooling. Even if this needs to be confirmed, we note the importance of independent observations, such as those supplied by GRACE and Jason-1.  Figure 3 : Upper curve : sea level rise from Topex and Jason altimetry (in blue) . |
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